I'm happy with my Pocophone F1. It's not the only phone with very good ROM/Free software support, but among the cheapest (around 300€, maybe a bit less by now, it should now also be available used).

It has lots of RAM, an SD card slot, a headphone jack, a high-end processor, a large battery and most importantly is very repairable (a screen costs about 20€).

The initial unlocking process is a bit annoying and stupid, but after that it's quite nice. The only thing you could say it really lacks is NFC.

The list of supported devices is here: https://wiki.lineageos.org/devices/

Looking at the LineageOS downloads page OnePlus has strong support (as a ratio of the number of phones they released to the number of phones that are compatible). Another idea is to check XDA Developers and see which forums are most active.

https://download.lineageos.org/

The OnePlus phones are good and they run on Lineage, but they do bundle google services by default. Of course, you can unlock the bootloader (wasn't too hard on the OP6, or the OPO before that, not sure about newer devices) and flash whatever you want, though, if you want to get rid of google services.

If you intend to flash custom rom on your phone, generally avoid devices with mediatek (or other non-qualcomm) chipsets because they tend to have higher risk of hard-bricking when you messed up which will require sending the phone to a service center to fix.

I just bought a oneplus pro 7 with lineage and microg and i love it !